Paper is manufactured as a continuous web formed on a papermaking machine. A paper web is formed at the so-called wet end of a papermaking machine by depositing a slurry of paper fibers and water onto a moving screen. The mat of fibers on the screen is first dewatered. The mat of fibers thus formed still consists primarily of water which is removed through progressive stages of pressing followed by drying on heated dryer rolls. The dried paper web may then be smoothed by passage through a calender or supercalender which compresses the web between opposed rollers and improves the surface finish and uniformity of paper thickness.
All the processes involved in papermaking, from the forming of the paper at the wet end to calendering at the dry end, are continuous in nature. However, the final step of the papermaking process, that of winding or reeling the paper web onto spools for removal from the paper machine, is an inherently discontinuous process and can result in non-uniform treatment of the paper web.
The reel of paper typically formed on a modern papermaking machine may have a diameter of 120 inches or more and a reel width of 200 to 400 inches. The paper on the so-called machine or jumbo reels is typically further processed by rewinding and slicing and sometimes coating the paper to form individual reels or sets to be used by paper-consuming customers such as newspapers. Studies performed in the past few years have shown that a paper web formed into the jumbo or machine reel can become damaged. The damage typically is in the form of tears near the edge of the sheet or creasing near the center. The damage typically results in the paper web breaking when it is further processed in a paper coating or rewinding machine or is utilized in a newspaper web press.
A set is a smaller roll of paper which has been formed from a jumbo reel. The last set is paper which is nearest the center of the original jumbo reel, that is the paper first wound onto the jumbo reel spool. Studies have shown that at some paper mills, as high as 70 to 80 percent of all rejects on critical paper grades are from the last set off the reel. In one study, 73 percent of the press room paper web breaks during printing were in the last set off the reel.
With increased papermaking speed and web widths, the size and weight of wound jumbos has also increased. In the past a certain percentage loss of paper due to reel defects was considered acceptable. However, with an increase in size of machine rolls, the problems associated with existing paper reels have been exacerbated while at the same time tolerance of product defects or waste of any kind has decreased due to increased competitive pressures and concern for maximum efficiency in the utilization of natural resources.
The solution to defects in the machine or jumbo reel is to produce a more uniformly wrapped paper web on the reel. The tightness or quality of the reel wrap depends on three factors: Tension, Nip pressure (including reel support for uniformity of nip pressure), and Torque. A paper winder employs a reel drum which is driven by the paper machine drive at a speed selected to impart a proper amount of tension. The tension is selected for a given grade and strength of paper and is typically 10-25 percent of the tensile strength of the given grade of paper. The web spool and the reel of paper built up thereon rides against the reel drum forming a nip therebetween. The nip compresses the paper which is wound onto the core. At the same time, at least initially, the reel drum may supply some support during initial winding for the reel.
Tension may be controlled by a centerwind assist drive which drives the machine reel. The centerwind assist is a differential torque drive with a differential torque controller controlling the amount of tension introduced into the web between the reel drum and the machine reel as it is built up on the core. German Application Number 91850261.8 entitled "Reel-up and Method for Regulation of the Nip Pressure in a Reel-up", corresponding to English language European Patent Application published as No. 0 483 093 A1, discloses a reel system which addresses some of the foregoing problems in the reel. The German Application discloses employing a tilting rail which is pivoted about the axis of rotation of the reel drum. Riding on the rail is the core, or reel spool, of the reel upon which the paper web is wound. The German Application discloses varying the angle between the rails and the horizontal such that the load of the nip formed between the machine reel and the reel drum remains uniform as the web is wound onto the machine reel.
Another type of winder system is the TNT System produced by Beloit Corporation which employs continuous control of the tension, nip and torque to produce machine reels of uniform density which are substantially less susceptible to the problems discussed herein. The Beloit TNT machine employs a horizontal rail located above a reel drum. The reel drum is vertically positionable and is controlled in response to a load cell which directly measures the nip pressure. The Beloit TNT machine solves the problems outlined above and produces a machine roll of uniform structure with minimal winder induced defects. Although the Beloit TNT machine provides a satisfactory solution to producing jumbo machine rolls of excellent uniformity, other approaches to the same problem are desirable. The papermaking industry has a large base of installed machinery of many differing makes and ages which are utilized to make a wide variety of papers and paper boards.
While completely new winder designs are proving effective at overcoming defects produced by winding a paper web onto a jumbo reel, there exists a large installed base of conventional Pope-type reels. In this type of reel a first arm positioned over a drive roll receives a spool. The first arm brings the spool into engagement with the drive drum where wrapping of the paper web is initiated. As the web is wrapped onto the roll, the first arm moves the spool down onto a pair of parallel rails which extend approximately parallel to the axis of the drive drum. The paper web is continuously wound onto the spool while it is held in engagement with the drive drum by a second arm. Such winders are exemplified, for example, in U.S. Pat. Nos. 3,743,199 to Carr et al and 3,857,524 to Melead et al. The Pope style winder controls the nip pressure between the reel and the winder drum through mechanisms which utilize the second winder arm to urge the reel against the winder drum. The need to improve uniformity of the paper wound onto reels requires attention to all three of the parameters influencing the wound roll uniformity and structure, namely the torque applied to the spool/core, the linear nip pressure between the wound web roll and the support drum, and the tension in the oncoming web being wound onto the roll.
Existing reels or winders control mainly the nip pressure between the wound web roll and the support drum. The employment of a centerwind assist drive in a conventional Pope style winder is difficult because the path taken by the spool/core from the first arm over the drum to the second arm and along the support rails is a path which experiences abrupt changes. This path complexity exacerbates or even causes the non-uniformity in the wound web, and makes it difficult to connect the centerwind assist drive in driving relation to the core as the paper web is built up on the core.
What is needed is a centerwind assist drive which can be employed with a conventional Pope style winder to improve the quality of the paper reels formed thereon.